1. Technical Field
The invention relates to a liquid crystal composition, a liquid crystal display device including the composition and so forth. In particular, the invention relates to a liquid crystal composition having a positive dielectric anisotropy and an active matrix (AM) device that includes the composition and has a mode such as a TN mode, an OCB mode, an IPS mode, an FFS mode or an FPA mode.
2. Background Art
In a liquid crystal display device, a classification based on an operating mode for liquid crystal molecules includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, a fringe field switching (FFS) mode and a field-induced photo-reactive alignment (FPA) mode. A classification based on a driving mode in the device includes a passive matrix (PM) and an active matrix (AM). The PM is classified into static, multiplex and so forth, and the AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth. The TFT is classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to a production process. A classification based on a light source includes a reflective type utilizing natural light, a transmissive type utilizing backlight and a transflective type utilizing both the natural light and the backlight.
The devices include a liquid crystal composition having a nematic phase. The composition has suitable characteristics. An AM device having good characteristics can be obtained by improving the characteristics of the composition. Table 1 below summarizes a relationship of characteristics between two aspects. The characteristics of the composition will be further described based on a commercially available AM device. A temperature range of a nematic phase relates to a temperature range in which the device can be used. A preferred maximum temperature of the nematic phase is approximately 70° C. or higher, and a preferred minimum temperature of the nematic phase is approximately −10° C. or lower. Viscosity of the composition relates to a response time in the device. A short response time is preferred for displaying moving images on the device. A shorter response time even by one millisecond is desirable. Accordingly, a small viscosity in the composition is preferred. A small viscosity at a low temperature is further preferred. An elastic constant of the composition relates to contrast in the device. In order to increase the contrast in the device, a large elastic constant in the composition is further preferred.
TABLE 1General Characteristics of Composition and AM DeviceGeneral CharacteristicsGeneralNo.of CompositionCharacteristics of AM Device1Wide temperature range ofWide usable temperature rangea nematic phase2Small viscosity1)Short response time3Suitable optical anisotropyLarge contrast ratio4Large positive or negativeLow threshold voltage and smalldielectricelectric power consumptionanisotropyLarge contrast ratio5Large specific resistanceLarge voltage holding ratio and largecontrast ratio6High stability to ultravioletLong service lifelight and heat7Large elastic constantLarge contrast ratio andshort response time1)A liquid crystal composition can be injected into a liquid crystal cell in a shorter period of time.
An optical anisotropy of a composition relates to a contrast ratio in the device. According to a mode of the device, a large optical anisotropy or small optical anisotropy, more specifically, a suitable optical anisotropy is required. A product (Δn×d) of the optical anisotropy (Δn) of the composition and a cell gap (d) of the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on a type of the operating mode. The suitable value is approximately 0.45 micrometer in a device having a mode such as the TN mode. In the above case, a composition having a large optical anisotropy is preferred for a device having a small cell gap. A large dielectric anisotropy in the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio in the device. Accordingly, the large dielectric anisotropy is preferred. A large specific resistance in the composition contributes to a large voltage holding ratio and a large contrast ratio in the device. Accordingly, a composition having a large specific resistance at room temperature and also at a temperature close to the maximum temperature of the nematic phase in an initial stage is preferred. A composition having a large specific resistance at room temperature and also at a temperature close to the maximum temperature of the nematic phase even after the device has been used for a long period of time is preferred. Stability of the composition to ultraviolet light and heat relates to a service life of the liquid crystal display device. In a case where the stability is high, the device has a long service life. Such characteristics are preferred for an AM device for use in a liquid crystal projector and a liquid crystal television and so forth.
A composition having a positive dielectric anisotropy is used for an AM device having the TN mode. A composition having a negative dielectric anisotropy is used for an AM device having the VA mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having the IPS mode or the FFS mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having a polymer sustained alignment (PSA) type. An example of the liquid crystal composition having a positive dielectric anisotropy is disclosed in Patent literature No. 1 described below.